The present invention relates to a hydrogen generation system. More particularly, the present invention relates to a method and system for by-product removal from a chemical hydride hydrogen generation system.
Fuel cells are seen as a promising alternative to traditional power generation technologies due to their low emissions, high efficiency and ease of operation. Fuel cells operate to convert chemical energy to electrical energy. Proton exchange membrane (PEM) fuel cells comprise an anode (oxidizing electrode), a cathode (reducing electrode), and a selective electrolytic membrane disposed between the two electrodes. In a catalyzed reaction, a fuel such as hydrogen, is oxidized at the anode to form cations (protons) and electrons. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. The electrons cannot pass through the membrane, and are forced to flow through an external circuit, thus providing electrical current. At the cathode, oxygen reacts at the catalyst layer, with electrons returned from the electrical circuit, to form anions. The anions formed at the cathode react with the protons that have crossed the membrane to form liquid water as the reaction product. Additionally, since the reactions are exothermic, heat is generated within the fuel cell. The half-cell reactions at the two electrodes are as follows:
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1) 
xc2xdO2+2H++2exe2x88x92xe2x86x92H2O+HEATxe2x80x83xe2x80x83(2) 
Various types of fuel cells have been developed employing a broad range of reactants. For example, proton exchange membrane (PEM) fuel cells are one of the most promising replacements for traditional power generation systems. PEM fuel cells comprise an anode, a cathode, and a proton exchange membrane disposed between the two electrodes. Preferably, PEM fuel cells are fuelled by pure hydrogen gas, as it is electrochemically reactive and the by-products of the reaction are water and heat. However, these fuel cells require external supply and storage devices for the hydrogen. Hydrogen can be difficult to store and handle, particularly in non-stationary applications. Conventional methods of storing hydrogen include liquid hydrogen, compressed gas cylinders, dehydrogenation of compounds, chemical adsorption into metal alloys, and chemical storage as hydrides. However, such storage systems tend to be hazardous, dangerous, expensive and bulky.
Other types of fuels have been proposed, including hydrogen-containing materials such as methanol. In some conventional systems, external reformers are employed to liberate hydrogen from the hydrogen-containing materials. The liberated hydrogen is then introduced into the fuel cell. However, the use of external reformers complicates the construction of the system, and results in a substantial loss in system efficiency. In other conventional systems, hydrogen-containing fuels may be supplied directly to the fuel cells, i.e. supplied unreformed to the fuel cell anodes. Once inside the fuel cell, the hydrogen-containing fuel may be directly oxidized or internally reformed, and subsequently oxidized to generate electricity. This occurs in some high temperature fuel cells, such as solid oxide fuel cells. These systems do not require a separate external reformer, and utilize fuels that are easier to handle than hydrogen. However, pure hydrogen typically offers better performance, and is generally more environmentally friendly than most hydrogen-containing fuels. Moreover, high temperature fuel cells operate at a minimum temperature of 600xc2x0 C. These high temperatures are required to reform the hydrogen-containing materials prior to carrying out the fuel cell reactions. As such, hydrogen-containing materials are generally unsuitable for conventional PEM fuel cells that typically operate around 80xc2x0 C.
Another method of generating and storing hydrogen has been recently proposed. This method uses a chemical hydride solution, such as NaBH4, as a hydrogen storage medium. Generally, chemical hydride reacts with water in the presence of a catalyst to generate hydrogen, as shown in the equation below:
NaBH4+2H2Oxe2x86x924H2+NaBO2+HEATxe2x80x83xe2x80x83(3) 
The chemical hydride solution acts as both the hydrogen carrier and the storage medium. Ruthenium, cobalt, platinum or any alloys thereof may be used to catalyze the above reaction. It is noted that hydrogen is liberated from both the sodium borohydride solution and the water. The sodium borohydride solution is relatively cheap, and is much easier and safer to handle and transport than liquid or pressurized hydrogen. As a result, there are some advantages associated with using sodium borohydride as a method of storing hydrogen as a fuel for use in fuel cells.
Known hydrogen generation systems typically employ a reactor to react chemical hydride with water in the presence of a catalyst to generate hydrogen. However, the by-product, in this example NaBO2, is less soluble then the reactant NaBH4. Specifically, NaBO2 is only approximately 20% soluble, whereas NaBH4 is approximately 40% soluble. Therefore, as hydrogen is generated, the concentration of NaBO2 in the solution increases until it reaches the solubility limit of NaBO2. If the reaction continues beyond this solubility limit, NaBO2 will precipitate out of the solution. The solid NaBO2 may clog the inlet and outlet ports of the reactor, thus impeding or blocking the flow of fluid through the reactor. In such instances, the hydrogen generation rate decreases significantly, and an insufficient amount of hydrogen is produced.
In some known systems, this problem is overcome by keeping the initial NaBH4 concentration lower than the solubility of NaBO2, that is, below 20%. However, this concentration is considerably lower than the solubility of NaBH4, and results in a limited hydrogen storage density. As such, these systems are generally not capable of responding in real time to the fuel (hydrogen) needs of the fuel cell. This ability is referred to as load following ability.
In other conventional systems, this problem is overcome by periodically replenishing the chemical hydride solution when the concentration of NaBO2 exceeds the solubility limit. However, this method is costly, wasteful, and environmentally unfriendly.
There remains a need for a chemical hydride hydrogen generation system that is adapted to reduce build-up of by-product in the chemical hydride solution. More particularly, there is a need for a chemical hydride hydrogen generation system in combination with a by-product removal system which is capable of responding in real time to the fuel (hydrogen) needs of the fuel cell.
It is an object of the present invention to provide a method and apparatus for improved by-product removal in a hydrogen generation system.
In accordance with a first aspect of the present invention, a method for removing a by-product from a chemical hydride solution is provided, where the by-product is produced in a reactor configured to contact the chemical hydride solution with a catalyst. The method comprising the steps of:
a) withdrawing at least a portion of the chemical hydride solution at a first temperature from the reactor;
b) cooling the portion of the chemical hydride solution to a second temperature below the first temperature, wherein a precipitate is formed from at least a portion of the by-product;
c) removing at least a portion of the precipitate from the portion of the chemical hydride solution;
d) heating the portion of the chemical hydride solution to a third temperature above the second temperature, wherein a remaining portion of the precipitate is dissolved in the portion of the chemical hydride solution; and
e) delivering the portion of the chemical hydride solution back to the reactor.
In accordance with a second aspect of the present invention, a system for removing a by-product from a chemical hydride solution is provided. The system comprises a circuit including:
a) a reactor including a catalyst for catalyzing reaction of the chemical hydride solution to generate hydrogen;
b) a pump for withdrawing at least a portion of the chemical hydride solution at a first temperature from the reactor and returning the portion of the chemical hydride solution to the reactor;
c) a cooling means for cooling the portion of the chemical hydride solution to a second temperature below the first temperature, wherein a precipitate is formed from at least a portion of the by-product, the cooling means being located in the circuit downstream of the reactor;
d) a separating means for removing at least a portion of the precipitate from the portion of the chemical hydride solution, the separating means being located in the circuit downstream of the cooling means; and
e) a heating means for heating the portion of the chemical hydride solution to a third temperature above the second temperature, wherein a remaining portion of the precipitate is dissolved in the portion of the chemical hydride solution, the heating means being located in the circuit downstream from the separating means.
Preferably, at least a part of the cooling means and at least a part of the heating means are provided by a heat exchanger, where the heat exchanger has one side located in the circuit downstream of the separating means and another side located in the circuit downstream of the reactor, thereby to transfer heat from the chemical hydride solution leaving the reactor to the chemical hydride solution flowing toward the reactor.